1. Field of the Invention
The present invention relates to a composite anti-friction bearing structure comprising a bearing substrate and an anti-friction layer. The bearing structure can be in the form of bushings, wear plates, wear rings, etc. The invention is particularly related to anti-friction bushings for use in die sets, presses and other heavy duty machinery.
2. Discussion of the Related Art
Bearing structures such as friction bearings, wear plates and wear rings are designed to accommodate rotational or linear contact motion in machines. In addition to lubricants, it is known to coat or plate various materials onto the bearing substrate to form an anti-friction layer providing reduced friction and increased wear resistance.
Long lasting precision bushings are an important component in commercially acceptable die sets. Because these bushings are often subjected to high press velocities and substantial side thrust forces, it is necessary that they be formed from a monolithic block and that they be carefully constructed to exacting specifications. Two types of plain guide bushings are well known to the art: hardened steel bushings and plated bushings in which a thin layer of bronze is plated inside the bore of a steel bushing. In either case, the bushing is closely fitted to a hardened and ground guide post with a diametrical clearance ranging from about 0.00076 to 0.002 cm.
Hardened steel bushings, when properly lubricated and maintained in alignment, will provide excellent service and wear life at moderate press speeds. For applications with higher speeds or higher side loads, bronze plated bushings are preferred because they reduce chances of galling or seizing. Bronze is also superior to steel in its ability to conduct heat away from the bearing surface.
These bronze plated bushings must be lubricated regularly to avoid failure. It is well known to provide bushings with means for lubrication, such as a lubricating fitting so that grease or other lubricants may be periodically introduced to the bushing internal bearing surface. However, under high velocity and extreme load conditions such lubricants are quickly dissipated; and if the tool operator is not diligent in the proper and periodic application of lubricant, it is possible that a bushing may seize despite all of the foregoing design precautions.
Many strategies have been employed to overcome the problem. One solution is to install a central lubrication system on each press which constantly meters lubrication to the guiding, but this approach is very expensive.
Another method for reducing servicing requirements is to employ a ball bearing guiding instead of friction guiding, but this too is expensive, and since ball bearing guiding is not as rigid as solid guiding, wear to the stamping tool is more rapid.
Some years ago a new friction-bushing was introduced to the market, one which incorporated a series of drilled holes filled with graphite plugs. The principle behind this design was that frictional heat will cause the graphite plugs to “sweat” and exude a portion of lubricant onto the bearing surface. This was the first step toward a self lubricating bushing, but the design had its problems.
First, the bushing ran at elevated temperatures, so the running clearance between the guide pillar and bushing had to be large. This made it impossible to perform close tolerance stamping work. Second, the multiplicity of lubrication holes weakened the bushing's structure, and wear was rapid. Third, the graphite lubricant left a dirty, greasy residue in the die area. Fourth, if the operator mistakenly added lubricant to the guiding, the graphite would form a sticky mess with the lubricant and make it very difficult to disassemble the guide pillar from the bushing. Fifth, the bushing was expensive and time consuming to produce. A series of holes had to be drilled into the bushing, then graphite plugs had to be inserted into the holes by hand.
A quality bushing for use in die sets and in other high load applications and capable of self-lubrication for extended periods of service is disclosed in U.S. Pat. No. 5,094,548 (Danly, Sr.). Danly, Sr. developed a process for forming a compacted and sintered porous bronze bearing layer on an internally machined cylindrical surface of the monolithic steel bushing body. For lubrication, the bushing included one or more recesses extending along the internal bearing surface and in communication with a passageway from a lubricant reservoir. This recess facilitated distribution of the lubricant on the internal bearing surface. The sintered bronze layer was preferably a porous layer impregnated with a solid polymeric lubricant.
Although the above bushing with sintered porous bronze bearing layer represented a dramatic improvement in the state of the art and required lubrication at much less frequent intervals, it nevertheless did require re-application of lubricants, and was liable to failure if not lubricated. Stamping plant managers have difficulty enforcing good maintenance practices on their production lines. One problem in particular stands out—getting press operators to regularly lubricate the stamping tools. This policing job is particularly difficult in large stamping plants running on a 3 shift basis.
It will be readily apparent that these shortcomings are not limited to bushings. Presently, “self lubricating” bronze plates are known, which are drilled and plugged with graphite plugs. These plates suffer the same limitations as discussed above with respect to bronze plated bushings. There is thus a need to improve bronze wear plates and wear rings of the type disclosed, for example, in U.S. Pat. No. 6,161,460 (Johnson et al) U.S. Pat. No. 5,372,026 (Roper), U.S. Pat. No. 5,865,054 (Roper) and U.S. Pat. No. 6,079,893 (Seidl et al.).
It is known from, e.g., U.S. Pat. No. 4,474,861 (Ecer) to provide a bearing structure having a substrate and a bearing surface of alternating hard metal and soft metal areas. Hard metals, commonly known in the art as hardfacing compositions, are generally either metal carbide based compositions or intermetallic hardfacing alloys. These materials are well known to those skilled in the art under various proprietary names, such as STELLITE alloys, HAYNES alloys, DELCROME alloys and TRIBALOY alloys. STELLITE alloys are examples of a carbide based hardfacing alloys, whereas TRIBALOY alloys are examples of intermetallic hardfacing alloys.
According to Ecer, powdered hardfacing composition is deposited on the bearing precursor surface, and then a concentrated beam of energy (from e.g., laser beam, electron beam, gas tungsten arc welding device) is applied to melt the hardfacing composition in the shape of strips, chevrons or islands. Excess, unsolidified hardfacing powder is removed, and a soft metal such as brazing and bearing metals and alloys (e.g., silver, silver based alloys, copper, copper based alloys, tin, tin based alloys, nickel, nickel based alloys, lead and lead base alloys, and aluminum bronze alloy) is melted to fill the gaps between the strips. The intermediate product is then machined to provide the final composite bearing surface.
Unfortunately, such a process and design cannot be easily adapted to non-planar surfaces, and particularly deep recesses such as internal bores of bushings. Further, the environment of use indicated in this patent, such as crankshaft bearings and “downhole” prospecting for oil, indicates presence of external liquid lubricants.
The present inventors thus determined that there is need for bushings and wear plates particularly suitable for use in high velocity and extreme load conditions where conventional lubricants are quickly dissipated and conventionally lubricated bearing structures are liable to failure if not properly monitored and replenished.
The present inventors wanted to develop a bushing that required no re-application of lubricant, that could operate under tight running clearances, would be long lasting and wear resistant, would not degrade in performance if not lubricated, and would be economical to manufacture.